As part of War in Heaven development we played with a number of experimental gametypes. Some are going in (in R1 or the future), some are more difficult to narratively justify.
One of those is one I'd like to work on as a standalone release at some point. Tower Defense!
Instead of buying towers, players could buy new fighters, bombers, and gunships to add to their defending swarm. Weapon and pilot skill upgrades could be purchased, individually or across classes of ships. Warships could be bought and positioned at preset warpin points, or moved between them with a special order system using query-orders and either a training message or reinforcements-based menu. You could even upgrade capship turrets, which could be really fun (upgrade her blob turrets to pulse weapons! Upgrade the flaks to standard flaks, then buy Fury AI for the ship!)
The goal would be to kill increasingly large and threatening waves of hostiles without being overrun. The player him/herself could be an invincible, stealthed, unarmed presence, or a 'boss' ship of some kind with a considerable tactical impact. You might be able to get a healing/repair system working based on the player's presence, for instance. If you disabled traitor, you could allow the player to use a negative damage primary to repair stuff, a sort of 'medigun'.
The resource system would (natch) be variable-based; kills would add to the resource pool, and calling in new types of ships and upgrades would deplete it. This system has already been implemented in BP2 and it's both modular and extensible.
I would like to give this a shot at some point in the future, if I don't end up using something of the type in a BP mission.